1. Field of the Invention
This invention relates to a nonvolatile memory device, in which conductive nano-material is used as a resistance change layer, and a method of fabricating the same.
2. Description of the Related Art
Recently, in accordance with the popularization of potable information terminals and information-digitalization, there is rapidly increased the demand for portable and large capacity of information recoding/reproducing apparatuses, i.e., memory devices. Specifically, a NAND type of flash memory device and a small HDD (Hard Disk Derive) are widely used because of increasing the memory capacity.
Further increase of the recoding density and the memory capacity is strictly required, and further shrinking, high-integration and high-speed, which are difficult to achieve in a conventional device, are desired. Recently, as an improved memory device, there is noticed a resistance change type of nonvolatile memory device, in which different resistance values are stored as data (for example, refer to non-patent documents 1 and 2).
The memory cell in the resistance change type of nonvolatile memory device is formed of a resistance change layer and electrodes sandwiching it. The resistance change layer stores one of two or more resistance states, for example, a high resistance state or a low resistance state, in a nonvolatile manner. Applying voltage higher than a threshold value, or current, charge, heat or the like to the memory cell, the resistance state is changed, and the changed resistance value will be stored as data. This data may be read out without breaking it.
Presently searched or developed as a resistance change memory material are binary metal oxide such as nickel oxide (NiO) and complex metal oxide such as strontium zirconium oxide (SrZrO3). However, with respect to the metal oxide, it is difficult to not only control composition and crystal structure thereof but also obtain desirable electrical properties with good reproducibility because the properties are unstable. Although the memory material suitable for the resistance change layer is presently searched, there is not yet found a suitable material.
Carbon-system material also is a candidate of the resistance change layer, and material search and forming method thereof are examined (for example, refer to non-patent document 3). Since the carbon-system material is formed of only carbon element, it is relatively easy to control the composition, and the process dependency is a little. Therefore, there is such a merit that the composition control is easy.
However, if not formed under a high temperature and a high pressure, the carbon film easily becomes black lead, i.e., graphite, and the electrical resistivity becomes too low. If the electrical resistivity is low, the operation current of the memory cell becomes large, and it results in that the power consumption becomes large. Additionally, it is considered that the resistance state change of the carbon film is due to the difference of the carbon-coupling state (i.e., 3p3 coupling or sp2 coupling). To change the carbon-coupling state, it is in need of carrying a large current. Therefore, there is such a problem that it is difficult to reduce the operation current.
On the other hand, carbon nano-material, which is a kind of carbon-system film, also is a candidate of the resistance change memory material (for example, refer to patent document 1). “Carbon nano-material” is a generic name of carbon material including carbon nano-tube, fullerenes and the like with a three dimensional structure in nano-scale size. This nano-material layer is different from a bulk film formed as a uniformly-dense film, and has a steric structure, i.e., a three dimensional structure, in which nano-size tubes are stacked in a state with many spaces contained, so that the current pass is limited spatially. Therefore, the current is small in spite of the conductive material, and it becomes possible to reduce the operation current of the memory cell.
In addition, although the detailed mechanism has not yet been found, it is considered that there is a resistance change operation based on “contact” and “non-contact” among nano-materials. This designates a possibility of reducing the operation current of the memory cell.
[Patent Document 1]    JPA-P2005-524000A
[Non-Patent Document 1]    P. Vettiger, G. Cross, M. Despont, U. Drechsler, U. Durig, B. Gotsmann, W. Haberle, M. A. Lants, H. E. Rothuizen, R. Stutz and G. K. Binnig, IEEE Trans. Nanotechnology 1, 39 (2002)
[Non-Patent Document 2]    P. Vettiger, T. Albrecht, M. Despont, U. Drechsler, U. Durig, B. Gotsmann, D. Jubin, W. Haberle, M. A. Lants, H. E. Rothuizen, R. Stutz, D. Wiesmann, G. K. Binnig, P. Bachtold, G. Cherubini, C. Hagleitner, T. Loeliger, A. Pantazi, H. Pozidis and E. Eleftheriou, in Technical Digest, IEDM03 pp. 763-766
[Non-Patent Document 3]    F. Kreupl, R. Bruchhaus, P. Majewski, J. B. Philipp, R. Symanczyk, T. Happ, C. Arndt, M. Vogt, R. Zimmermann, A. Buerke, A. P. Graham and M. Kund, in Technical Digest, IEDM08 pp. 521-524